The present invention relates to control circuitry for cathode ray Tube (CRT) displays, and in particular, to bias control circuitry for controlling CRT beam currents.
As is well known, a conventional CRT produces images on a display screen coated with phosphors by focusing beams of electrons onto the screen and making the phosphors emit visible light. The electron beams are typically formed by causing a cathode inside the evacuated CRT to emit electrons by applying different voltages between the cathodes (e.g., one for each of the primary colors red, green and blue) and control grids. One or more anodes in the CRT attract, focus and accelerate the electrons. Hence, the anodes may be used to collimate the electrons into tight beams hitting appropriate spots of the display screen. Additionally, coils located outside the CRT are driven with currents to create magnetic fields within the CRT to direct the electron beams to the desired spots of the display screen.
As the CRT phosphors respond to the varying intensities of the beam currents, the brightness of the picture becomes a function of the beam currents. However, over the lifetime of the cathodes, the respective beam currents do not stay constant. As a result, the picture intensity will generally change over time, notwithstanding the consistency of the voltages applied to the cathodes. Various compensation systems and techniques have been used and proposed to compensate for changes in beam currents. For example, one such system and method is described in U.S. Pat. No. 6,771,023, the disclosure of which is incorporated herein by reference.